1. Field of the Invention
The present invention generally relates to a mobile communication system, and in particular to an apparatus and method for performing channel compensation in a mobile communication system.
2. Background of the Related Art
Broadband mobile communication systems use STTD (space-time transmit diversity) as a method for increasing forward capacity. In this method, two symbols output from a modulation block are converted through time-space coding so as to have the same information but cross each other. The two symbols are then respectively transmitted to two antennas, and the transmitted symbols are demodulated by compensating channel phase distortion of signals passing different radio channel paths by antennas in a rake receiver of a mobile station. In demodulating the received signal, it is important to accurately predict channel phase distortion of the signal passing through each channel path, because the prediction accuracy affects the performance of the whole system.
In order to predict channel phase distortion, a base station transmits data with a pilot signal as a pre-appointed data pattern, and the mobile station receives the pilot signal and extracts channel distortion information.
FIG. 1 is a block diagram illustrating a rake receiver of a broadband mobile communication system in accordance with the conventional art. This receiver includes a descrambler 4 for descrambling a signal received through an antenna using a scrambling code, a reverse channelizer 5 for dividing an output signal of the descrambler 4 by channels using a spreading factor, a channel predictive unit 6 for predicting phase distortion of each channel from the output signal of the descrambler 4, and a demodulator 7 which compensates for channel distortion of an output signal of the reverse channilizer 5 based on the phase distortion output from the channel predictive unit.
Operation of the conventional rake receiver will now be described. A forward link signal transmitted from the base station is allocated to each finger of the rake receiver. The allocated signal is synchronized with each multi-pass position output from a cell seeker (not shown) and is correlated by symbol units using a scrambling code and spreading factor. More specifically, the descrambler 4 descrambles the received signal using the scrambling code output from a code generator (not shown). The reverse-channelizer 50 divides the output signal of the descrambler 4 by channels with the spreading factor output from the code generator (not shown), and the channel predictive unit 6 predicts a channel phase distortion value of each channel (antenna 1 and antenna 2) from the output signal of the descrambler.
The demodulator 7 compensates the phase distortion (channel path distortion) of the output signal of the reverse-channelizer based on the channel phase distortion value output from the channel predictive unit, decodes it, and outputs a symbol signal.
FIG. 2 illustrates the channel predictive unit of the rake receiver shown in FIG. 1. As Shown, the channel predictive unit includes a first antenna mixer 10 for multiplying a scrambling code (Csc) by a signal input through the finger and dividing the signal into I and Q signals, an I-channel processor 12 for processing the I signal output from the mixer 10 and generating a channel phase distortion value for the I channel, and a Q-channel processor 14 for processing the Q signal output from the mixer 10 and generating a channel phase distortion value for the Q-channel. The channel prediction unit further includes a second antenna mixer 20 for multiplying the scrambling code (Csc) by a signal received through the finger and dividing the signal into I and Q signals, an I-channel processor 22 for processing the I signal output from the mixer 20 and generating a channel phase distortion value about the I-channel, a Q channel processor 24 for processing the Q signal output from the mixer 10 and generating a channel phase distortion value about the Q channel. Herein, the I channel processor 22 and the Q channel processor 24 for antenna 2 each include a mixer for performing a mixing operation based on a symbol pattern A, however the operation of the rake receiver is not much effected by that.
For convenience purposes, operation of only the I-channel processor 12 will be described.
First, a signal received through the finger (Ri and i are fingers) is divided into an I signal and a Q signal in mixer 10, and the I signal and the Q signal are transmitted to the I channel processor 12 and the Q channel processor 14, respectively. Mixers 50, 60 of the I and Q channel processors respectively multiply the I and Q signal by a channelizing code (Cch) in order to divide the signals by channels, and adders 52, 62 respectively divide the channelized signals into channels having the same phase.
However, as depicted in FIG. 3, a symbol pattern group output from antenna 1 and antenna 2 of the transmission block (base station) consists of 4 symbols per each frame. In this case, “A” means 1 symbol and it is maintained for 256 chip. Accordingly, accumulators 54, 64 respectively accumulate each output signal of the adders 52, 62 for 4T (T=1 symbol cycle), calculate a moving average, and output a channel phase distortion value for the I channel and the Q channel of antenna 1. Here, it is assumed there is no channel variation for 4T. The same operation is applied to the I channel processor 22 and the Q channel processor 24 of antenna 2.
In the conventional art rake receiver, by accumulating-adding a channel signal allocated to each finger in a certain interval (4T), a channel phase distortion value for the antennas 1, 2 is output. In general, a fading phenomenon is affected by a natural phenomenon and artificial constructions. It occurs on transmission paths varied every time interval. In addition, when a mobile communication terminal is moved between base stations, a frequency of a radio wave proceeding to the front increases, and a frequency of a radio wave proceeding to the rear decreases by the Doppler effect. This fading phenomenon means that reception-level variation caused by amplitude and phase distortion of a radio wave occurs in reception and combination thereof, and this lowers transmission quality.
For example, when a terminal is in a hand over state, a signal allocated to each finger has a different Doppler frequency according to a location of a base station. Accordingly, each signal has a different phase and amplitude. In addition, when the mobile communication terminal is moved between the base stations, a transmission signal is transmitted to each finger through independent multiple paths different from each other.
In order to improve transmission quality of a terminal, the fading phenomenon and Doppler frequency have to be compensated for by performing channel prediction differently based on a traveling speed of a terminal, a location of a base station, and multiple paths. However, in the conventional art, by calculating a channel distortion value by only calculating a moving average for a fixed interval (4T) in each finger regardless of the traveling speed of a terminal, there may be an error in a channel size distortion value and a channel phase distortion value. Accordingly, the performance of the terminal may be lowered using the conventional art method.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.